416 



MOLLUSCA. 



The polypide of a Polyzoon (fig. 260, 2) consists essentially 

 of a double-walled sac, filled with fluid, and perforated by 

 an aperture where the mouth of the polypide is situated. In 

 the majority of cases the outer wall of the sac (termed the 

 " ectocyst ") is of a horny consistence, or may be more or 

 less highly calcareous. It forms a little chamber, which is 

 technically called the " cell." At one point, varying in its 

 position, the cell is furnished with an aperture or " mouth " 

 (fig. 260, 1), whence the polypide can protrude its tentac- 

 ulate head. The inner wall of the sac (termed the " endo- 

 cyst ") is invariably flexible and membranous, and the space 

 included within it is filled with fluid, in which floats the 

 alimentary canal. The commencement of the alimentary 

 canal is surrounded by a series of hollow ciliated tentacles, 



Fig. 260. Morphology of Polyzoa. 1. Portion of the coenoecium of Flustra truncata, magni- 

 fied. 2. Diagram of a Polyzoon (after Allman): a, Region of the mouth surrounded by 

 tentacles ; 6, Alimentary canal ; c, Anus ; d, Nervous ganglion ; e, Investing sac (ectocyst) ; 

 /, Testis ; /', Ovary ; g, Retractor muscle. 3. Bird's-head process, or " avicularium," of a 

 Polyzoon. 



which are mostly arranged in a circle in the marine Polyzoa, 

 but are disposed in the shape of a horse-shoe in most of the 

 fresh-water forms. The digestive canal passes through the 

 body-cavity, without opening into it, and terminates in a 

 distinct anus placed near the mouth. The only other organs 

 possessed by the polypide are a nervous ganglion, and the 

 organs of reproduction, each zooid being hermaphrodite. 



